This invention relates to semiconductor devices, particularly to those of the class employing nitrides or nitride-based compounds as semiconductors, as typified by metal semiconductor field-effect transistors (MESFETs) and high electron mobility transistors (HEMTs), and more particularly to means in such semiconductor devices for reduction of leakage current to a minimum.
The gallium-nitride-based semiconductor devices have been known and used extensively which have the gallium-nitride-based semiconductor region formed on a sapphire substrate via a buffer region. The sapphire substrate had, however, a weakness of being difficult of dicing for its hardness, in addition to being expensive. These shortcomings of the sapphire substrate are absent from the substrate of silicon or silicon compound suggested by Japanese Unexamined Patent Publication No. 2003-59948.
As taught also by the unexamined patent application above, a multilayered buffer region is interposed between the silicon or silicon-based substrate and the nitride-based semiconductor region (hereinafter referred to as the main semiconductor region) constituting the main working part of the semiconductor device. The buffer region is in the form of alternations of two different kinds of buffer layers such for instance as AlxGa1-xN, where the suffix x is a numeral that is greater than zero and equal to or less than one, and GaN. The multilayered buffer region has proved conducive to improvements in crystallinity and flatness of the main semiconductor region grown epitaxially thereon.
There have, however, been some problems left unresolved with the gallium-nitride-based semiconductor devices of the kind having the multilayered buffer region on a silicon substrate. The gallium-nitride-based main semiconductor region on the multilayered buffer region has a thickness limited by the spacing between the drain and source electrodes formed on the main semiconductor region. For example, for an interelectrode distance of five to 20 micrometers or so, the thickness of the main semiconductor region is only about 0.2 to 3.0 micrometer. Additionally, due to the heterojunctions between the AlxGa1-xN layers and GaN layers of the multilayered buffer region, what is known to the specialists as two-dimensional electron gas is generated in the GaN layers, with a consequent drop in the resistance of these layers. What is more, being so thin, the AlxGa1-xN layers are easy to give rise to the quantum-mechanical tunnel effect.
Let us suppose for instance a gallium-nitride-based HEMT of the above outlined prior art design. Leakage current (indicated at IA in FIG. 1 of the drawings attached hereto) was easy to flow between the drain and source of the HEMT via the multilayered buffer region and silicon substrate during the nonconducting periods of the device when the channel layer overlying the electron transit layer of the main semiconductor region was closed by the depletion layer under the gate. The leakage current caused a rise in the potential of the silicon substrate, with a corresponding increase in potential difference between the substrate and the source. The increased potential difference caused electric field concentrations on the sides of the substrate, buffer region, and main semiconductor region. It must also be taken into consideration that the sides of the substrate, buffer region, and main semiconductor region are not necessarily good in crystallinity, being both exposed and affected by dicing. The HEMT was therefore susceptible to breakdown as a result of field concentrations at the sides of the buffer region and main semiconductor region.
Moreover, in addition to the noted leakage current between drain and source via the substrate and buffer region, there was another path for current leakage (indicated at IB in FIG. 1) along the relatively low-resistance sides of the HEMT or like semiconductor device. Thus an inconveniently large amount of total leakage current (sum of IA and IB) existed between drain and source. The usual practice in the semiconductor industry is to assess the drain-source voltage-withstanding capabilities of HEMTs and like devices in terms of the magnitude of current leakage. The larger the amount of current leakage in each such device, the lower is the assessment of the voltage-withstanding capability of that device. Leakage current is itself objectionable because it may lead to the breakdown of the device.
The foregoing difficulties are most pronounced in conjunction with the gallium-nitride-based semiconductor devices of the kind having their main semiconductor regions formed on silicon substrates, which are low in electric resistance, via the multilayered buffer regions. The same problems may, however, manifest themselves with gallium-nitride-based semiconductor devices in which the main semiconductor regions are grown on sapphire substrates via a low-temperature-grown buffer region, as well as those using silicon carbide substrates. Not only HEMTS, moreover, but other semiconductor devices having at least two electrodes directly overlying the main semiconductor region, too, are likely to suffer similar difficulties.